1. To create a tarfile containing several files:
tar cvf data.tar *.dat
This creates a tarfile data.tar containing all the files in the current directory which have the filename extension .dat.
Look at the manual page for details of the command options cvf.
2. To create a tarfile of a directory:
tar cvf project.tar Project Project/README Project/Makefile Project/Makefile Project/main.c Project/screen.c Project/menus.c Project/calculator.c Project/setup/configuration
This creates a tarfile project.tar which contains the directory Project and its contents.
3. To create a tarfile containing several directories:
tar cvf book.tar text illustrations
This creates a tarfile book.tar containing the directories text and illustrations.